Hello Jazzy ,
As to your instrument in question , as far as the EQ goes , I would have to have a close look with ear and eye . The way that you describe it it seems perhaps like the midpoint on each EQ potentiometer could be considered 0 or flat and plus and minus adjustment in either direction respectively for Bass , Midrange ,and Highs .
I have 2 vintage original green face Furman PQ-3 three band Parametric EQ's in a rack complete with an Alembic F-2B Preamp . On the three Band Furman PQ-3 The controls left to right are gain level , and then the same for each of the 3 respective bands ( LOW,MID,HIGH) Frequency_____Bandwidth________Plus/Minus Equalization. I believe Phil Lesh , Jerry Garcia and Bob Weir had similar EQ signal processing in one of their various configurations of equipment in the 1970's . I think Jerry purchased one of the earliest Furman PQ-3's .
As far four band Parametric EQ there would be three separate potentiometers for each range as well; My Neptune Electronics 342 Parametric has Low ,Low Mid,High Mid & High bands .Each band would have Frequency,Band Width & Gain Potentiometers. A total of twelve potentiometers per audio channel just for EQ . I use that one in my studio playback setup along with a 31 band third octave Graphic EQ set up with an RTA ( real time analyzer). And there is also the dual channel rack mount oscilloscope from HP ___Yes, I am a gear geek _____.
I hope that anything that I wrote was of help.
Wolf
